If the first argument is a number, it is treated as a version number. If
the version of Perl is less than version, an error message is
printed and Perl exits. This provides a way to check the Perl version
at compilation time, instead of waiting for runtime.

If version appears between Module and list, then use
calls the version method in class Module with
version as an argument.

Otherwise, use imports some semantics into
the current package
from the named Module, generally by aliasing certain subroutine or
variable names into your package. It is exactly equivalent to the following:

BEGIN { require Module; import Modulelist; }

The BEGIN forces the require and import to
happen at compile time. The require makes
sure that the module is loaded into memory if it hasn't been yet.
The import is not a built-in function - it's just an ordinary static
method call into the package named by Module to tell
the module to import the list of features back into the current package. The
module can implement its import method any way it likes, though most modules
just choose to derive their import method via inheritance from the Exporter
class that is defined in the Exporter module.

Because this is a wide-open interface, pragmas (compiler directives)
are also implemented this way. See Chapter 8 for descriptions
of the currently implemented pragmas.
These pseudomodules typically import semantics into the current block scope, unlike
ordinary modules, which import symbols into the current package. (The
latter are
effective through the end of the file.)

There's a corresponding declaration, no,
that "unimports" any meanings
originally imported by use, but that have since become less important: